Information On Double Depth As On A Fishtail Gauge

[Pmedic828]

Can someone explain to me how to use the numbers that are printed on a fishtail gauge used for single point threading? From what I can understand, the double depth is listed. Are these numbers used to determine how much you would need to turn in the compound at 30 degrees? Are these numbers direct reading or do you have to take the cosine of 60 degrees and multiply this by 1/2 the double depth number on the gauge?
Please explain what these numbers mean and how to use them to a novice like myself. Thanks for your time.

 

[randyc]

Forget about those numbers - they're only useful if you use the cross-slide to adjust the DOC. As you alluded, the compound (adjusted to slightly less than thirty degrees) is the preferred option. Yes, you can use the trigonometric correction but I would still have a slight doubt about the pitch diameter that was produced. I would have the same reservation if the cross-slide was used to adjust DOC !

If you single-point much, invest in a thread micrometer. They are a little pricey and have a learning curve but eventually save a LOT of time (and scrapped parts). Frankly, making a chasing head - doesn't have to be very sophisticated - is so much more convenient and produces better threads than single-pointing. Useful up to about 3/4 - 10 threads. You will see one that I made as well as a couple of modified, inexpensive commercial ones here:

http://www.hobby-machinist.com/threads/lathe-tailstock-tooling.32941/#post-278063

 

[machinistmarty]

As stated above that is double depth straight in (using cross slide) The formula that i was taught(for using compound on 30) and have always used is .750 times number of threads . Thats per side, but alot of compounds feed in double so check your lathe. For example the double depth for 12tpi on your centergage should be .108, ok .750/12= .062 per side or.125 double depth on compound. Remember thats just approx. always check your thread when getting close.

 

[Halligan142]

While all above is correct don't discount those numbers on the gauge completely. They're good for cutting your thread reliefs to the correct depth.

 

[machinistmarty]

I have been doing this for a long time. Things that ppl do on this site still amaze me. I dont really know why the formula for feeding on the compound works, but it does. My mentors 30 years ago didnt explain it, just told me it worked and by god they were right. If you trig it, it dosent really make sense. should be .866. but that dosent account for the .5 on the face if you do the cosine. Something to do with a compound angle I suppose. If you didnt see my above post it was .750 divided by number of threads, multiply by 2 for double depth. I watched a youtube video recently where someone had a chart for feeding in on the compound. Same as i was taught once I did the math. I was fortunate to have some really talented guys to help me when I was an apprentice. They scoffed when we got our first digital readout on a mill. But I tell ya, these guys could do some amazing work.

 

[higgite]

The difference is the type of thread. Some fishtails show double depth for sharp "V" threads and some for American National threads. Use .866 for V threads and .75 for Amer. Nat'l.

Tom